This invention relates to a gas monitor that is used to measure the concentration of a particular constituent of a gaseous medium. More particularly it relates to a monitor that periodically recalibrates itself by measuring standard samples of known concentration and adjusting the monitor output accordingly.
Monitors of the type with which we are concerned are used to monitor selected areas for the concentrations of particular vapors or gases. For example, the workers in a manufacturing plant may be subjected to a toxic gas as the result of leakage from containers, pipes, or equipment in which the gas is used. A gas monitor is therefore installed to sense the atmospheric concentration of the gas in an area in which the leakage might occur and to sound an alarm when the concentration approaches a safe limit. A typical monitor comprises (a) a sensor which samples the atmosphere in the area under surveillance and provides an electrical signal corresponding to the atmospheric concentration of the toxic constituent, (b) a processor that processes the sensor signal and provides a corresponding output signal and (c) a indicator that converts the output signal to a visible or audible output.
The sensor used in a gas monitor is subject drift, i.e., changes in the correspondence between the sensor signals and the concentration of the monitored gaseous constituent. Accordingly, the monitor must be recalibrated periodically to maintain the accuracy of its output. In essence, calibration is accomplished by exposing the sensor to an atmosphere having a known concentration of the monitored constituent and adjusting the signal processor circuitry to make the monitor output correspond to that concentration. The procedure usually involves two standard concentrations. One of these is zero concentration and the other is a known finite concentration. In the instrument art the corresponding calibration adjustments are called "zero" and "span" adjustments.
The present invention is directed mainly to the span adjustment of a gas monitor. This adjustment has heretofore been made essentially by adjusting the setting of a potentiometer which controls the gain in an amplifier circuit. This is generally satisfactory for manual calibration, but where automatic calibration is desired, it requires the use of a servo motor to drive the potentiometer. This increases overall expense and makes for a more cumbersome device. Also, if the system is recalibrated often, as it should be for maintenance of accurate readings, reliability suffers because of wear in the potentiometer.
Accordingly, it is an object of this invention to provide a signal processor in the monitor that automatically accomplishes recalibration, including span adjustment, without the use of moving parts.